The power consumption increases exponentially day by day which invites an urgent requirement of cost-effective techniques for generating electricity from various water resources. The flowing water in manmade canals, tailraces, diversion channels, or other fluid flow channels is considered as a reliable resource for generating electricity up to few megawatts. Even though there is a lot of subcritical manmade canals that are useful for generating hydrokinetic power, this potential for power generation remains untapped, since challenges and limitation to enhance velocity in a subcritical manmade channel. One of the key reasons is that the manmade channels are mild slope channels.
The subcritical channel water conveyance system includes closed channel water conveyance system and open channel water conveyance system. In open channel water conveyance systems, gravity is the driving force that moves the water, while frictional forces along the wetted perimeter of the channel oppose the motion. A slight downward slope is sufficient to overcome the opposing frictional forces. The bottom or sides of a flow channel is either unlined or lined with suitable materials. If the flow channel is lined, the flow retarding frictional forces are reduced. The overall flow velocity in open channels is usually designed to be relatively small to prevent turbulent flow conditions in the channel or to prevent scouring along the wetted perimeter of the channel. The ambient flow velocities in open fluid flow channels are generally less than 1.5 m per second.
In conventional methods of the generation of hydrokinetic power, the power generation systems are within the existing channel system. In conditions, it is ensured that the sufficient flow velocities are available naturally in the system. Otherwise, the velocity of the water flow is enhanced by inserting flumes in the existing channel without diverting water. There are many challenges in subcritical channels to enhance the velocity. The flow in subcritical channel is accelerated to a higher velocity than available with the expanses of depth of channel so that the swept area for power generation is reduced and it suits only for vertical axis turbines. The theoretical limit for power coefficient for these turbines is 0.59. This limit is known as Betz limit. However, the power coefficient of these turbines in practical conditions ranges from 0.30 to 0.35. In such channels the velocity of water can be raised only up to a critical depth of the channel. Beyond that the upstream of the channel get affected. Generally in the subcritical channel, the acceleration is done by flumes so that the velocity at the throat is more and suits for power extraction. However, the length of the throat is very small in normal condition and can accommodate one or two turbines for power generation. This limits the maintenance of higher depth and velocity for longer length for high power generation.
Subcritical channels are mild slope channel generally have higher depth and lesser velocity. Therefore, it is not possible to have higher velocity for the flow at higher depth. In order to have higher velocity, the depth of channel is to be reduced. This kind of channels generally not suitable for horizontal axis ducted turbines which is required both higher depth and higher velocity.
Hence, there is a need of a method of generating hydrokinetic power from subcritical channel where natural velocities not available for generating power to cater the rising demand for power. Additionally, there is also a need for cost effective extraction of hydrokinetic power from channels, especially in manmade conveyance systems. Since the primary function of these water conveyance systems is for drinking or irrigation purposes, it is imperative that the installation and operation of hydrokinetic power generation facility in these subcritical channels do not have an adverse impact on their capability to deliver water in a desired flow and at an expected quality.